About ten years ago the writer made his third attempt to correlate the chemical composition and optic properties of the lithia micas. At that time he wrote: “An attempt to solve this problem (the formulas of these micas) has met with little success,” and “some examples do not agree well with the diagram. Apparently these micas are dimorphous, and that condition would doubtless entail variations in optical properties, the extent and character of which are at present unknown.” Since then he has continued work on the problem and has obtained the kind cooperation of several colleagues who have generously supplied samples from various localities, in some cases, of analyzed material. It is a pleasure to acknowledge the assistance of Dr. Harry Berman of Harvard University, Dr. W. F. Foshag of the United States National Museum, Miss Jewell Glass, Dr. W. T. Schaller, and Dr. R. E. Stevens of the U. S. Geological Survey, Edward S. Simpson, Government Mineralogist of Western Australia, Dr. C. H. Stockwell of the Geological Survey of Canada, and H. Gordon Taylor of Boston, Mass.

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